The purpose of this research study is to evaluate the effects of antibiotics on the absorption of mycophenolic acid, also known as mycophenolate mofetil (Cellcept), a medication used to prevent organ rejection in patients receiving various organ transplants. Cellcept's absorption partly depends on enzymes produced by bacteria found in the large intestine. When antibiotics are given to patients in the hospital, these bacteria are temporarily disrupted and may not produce enough enzyme to help the body absorb Cellcept. If this happens the concentration of Cellcept may not be high enough in the blood to prevent the body from rejecting the newly transplanted organ. In this study we are interested in determining the effect of several different antibiotics on the absorption of Cellcept.